Devices are commonly installed in fluid flow systems to measure a differential in pressure across a piece of equipment. Based on the known characteristics of the equipment, the differential pressure can be converted into a flow rate. 
A common device for measuring pressure is a pitot tube. The pitot tube is positioned at the outlet side of a nozzle to measure the pressure at a specific point of discharge. The opening of the pitot tube is placed in the stream of the fluid flow and positioned such that the tube is located in the exact center of the stream, one half of the nozzle outlet diameter away from the outlet of the nozzle and parallel to the flow path. Usually incorporated with the pitot tube is a gauge.  
In a pitot tube, static pressure, which is atmospheric pressure in an open system, is compared to total pressure. Using Bernoulli's Equation, the pressure differential is then translated to a flow velocity. A gauge device may be graduated such that the readout is directly displayed as a velocity. 
Pitot tubes are commonly used where there is a desire to determine the flow rates of fluids for equipment testing. Fire pumps, in addition to other pumping equipment, are required to meet certain specifications. Therefore, an accurate method of testing is important. 
Many fire hydrant flow tests are conducted by taking a pitot reading directly from the nozzle on the fire hydrant. Due to inexact orifice diameter, excessive turbulence, which causes gauge bounce of +/-10 psi, and incorrect pitot positioning, these options give the least dependable readings. 
Some devices integrate the pitot tube with the fluid flow piping. A general problem with pitot tubes is that they are difficult to position in the center of flow. To get an accurate reading, the pitot tube must be placed in the center of flow, in a position one half the nozzle outlet diameter away from the outlet of the nozzle, and parallel to the flow. The more removed from these requirements, the less accurate the measurement will  be. Also, pitot tubes can easily be damaged by solids that may be intermixed with the fluid. 
U.S. Pat. No. 4,555,952 to Jenkins, issued on Dec. 3, 1985, discloses a differential pressure sensor. The pressure sensor responds to pressure differential across an orifice of known size. Here, the fluid pressure is measured on one side of an orifice plate. The fluid flows through the orifice. The pressure is also measured on the other side of the orifice plate. The two pressures are compared across a diaphragm. The pressure differential is transmitted to an electronic transducer located a distance safe enough away to protect the transducer from the temperature of the fluid. Thus, the '952 patent requires an additional energy-consuming and turbulent-producing orifice plate device to measure pressure differential which can affect the accuracy of the measurement, while the present invention does not. The present invention compares the internal system pressure as the fluid passes through a constant pressure nozzle, which results in less turbulence, with the pressure of the atmosphere. The difference in pressure is translated into a flow rate for determining equipment performance. 
U.S. Pat. No. 4,343,193 to Holden, issued on Aug. 10, 1982, discloses a flow measuring apparatus. The differential in pressure is measured across a removable orifice plate. When a  reading is not required, the orifice plate can be removed from the system piping. Again, the present invention does not require an additional energy-consuming, turbulent-producing orifice plate device to measure pressure differential. 
U.S. Pat. No. 2,564,272 to Morton, issued on Aug. 14, 1956, discloses a flow meter attachment for hose nozzles. The nozzle is accompanied with a gauge tapping for a pressure gauge. The attachment consists of two tubular sections. The inner tubular section has a plurality of holes which extends through the wall such that there is fluid communication between the inside and outside of the inner tubular section. The gauge measures the pressure differential between the two tubular sections and the atmosphere. However, this does not allow for full measurement of the velocity component of Bernoulli's Equation since there is diminished flow in between the two tubular sections, thereby resulting in a less accurate reading over the present invention. In addition, the '272 patent requires that the characteristics of the nozzle be known, such that the pressure gauge can be calibrated directly in fluid flow. However, the present invention uses a constant pressure design nozzle which provides a more accurate gauge reading. Also, a constant pressure nozzle produces negligible turbulence, thereby providing for nearly perfect laminar flow. Hence the resulting  flow will result in less needle bounce and a constant steady readout of pressure as measured by a gauge. 
The present invention provides a more accurate reading of fluid flow through the use of a pressure gauge in conjunction with a constant pressure nozzle of known characteristics, wherein the measuring gauge does not interfere with the dynamic profile of the fluid flow. 